TWI434290B - A zirconium alloy that withstands shadow corrosion for a component of a boiling water reactor fuel assembly, a component made of the alloy, a fuel assembly, and the use thereof - Google Patents

Description

Shading resistant zirconium alloy for components of boiling water reactor fuel assembly, components made from the alloy, fuel assembly and their use

This invention relates to the field of nuclear reactors and, more particularly, to zirconium alloy components for assembling fuel assemblies for boiling water nuclear reactors (BWR).

Zr alloys are widely used in the fuel assembly of nuclear reactors for the manufacture of components that are subject to severe conditions such as radiation, mechanical stress and corrosion. These components include cladding fuel pellets, tanks, grids, various spacer elements, and the like.

Various types of Zr alloys have been developed, which correspond to various requirements of the user, and depend on the desired properties of the various components. These are determined by mechanical, thermal, and physicochemical (radiation, corrosion) stresses such as those encountered when they are used in the reactor.

Some of these alloys are characterized by a significant amount of Nb. A general description can be found particularly in U.S. Patent No. 4,490, 223, in which they are used to construct a tube for use in a light water reactor, including both a boiling water reactor (BWR) and a pressurized water reactor (PWR).

Other documents (US-A-5 266 131) propose to apply them to components manufactured from sheet materials. However, to date, industrial applications of such Nb-containing alloys have been limited to pressurized water reactors (PWR). Attempts to use the same alloys in boiling water reactors (BWR) have not been finalized because they are unqualified in general corrosion and in nodular corrosion. Therefore, a common method in BWR is to use other types of Zr alloys.

A fuel assembly assembly is proposed in the document JP-A-62 182 258, in particular for BWR, from Zr-Nb-Sn-Fe-O alloy, via cold rolling, followed by β (or α+β) quenching, and then processing. Harden at least 30% and then age at temperatures greater than the recrystallization temperature (e.g., 450 ° C to 550 ° C) without subsequent cold rolling. This produces a structure having fine precipitates of β Nb and ZrFe ₂ intermetallic compounds. The concept that emerged was to obtain components that were relatively insensitive to nodular corrosion and that had high toughness and ductility.

A recent proposal (document WO-A-2006/004499) is the use of Nb-containing alloys in BWR to fabricate components from sheet metal. There is no alloying element content of 1.6%. The thermomechanical treatment of the alloy results in the conversion of substantially all of the secondary phase particles into beta Nb particles containing at least 90% Nb. Preferably, the Fe content of the alloy is in the range of from 0.3% by weight to 0.6% by weight, and the alloy contains only a significant amount of Sn in addition to Zr, Nb, and Fe. The content of any other alloying element must not exceed 500 parts per million (ppm). These alloys attempt to provide good resistance to conventional types of corrosion and radiation growth.

However, a problem often encountered in BRW is also associated with the appearance of so-called "shadow corrosion."

This is a type of corrosion that occurs when two components made of different types of materials are connected by galvanic current in the presence of an oxidizing species (electrons are immersed between two materials in a medium exhibiting non-zero conductance) transmission). In particular, the conductive medium is the boiling water of the reactor. Localized white corrosion can be observed when a Zr alloy component, such as a box or a combustion cladding, is joined to an assembly made of a Ni-based alloy or stainless steel, such as a grid used to separate the tubes. Now on the Zr alloy, on the surface corresponding to the shadow of other parts made of Ni-based alloy or stainless steel. This phenomenon is amplified by irradiation, which modifies the physicochemical properties of the material and, in addition to the species created by the oxygen dissolved in the boiling water of the reactor, is also formed on the surface of the assembly by radiolysis of the heat transport fluid. Oxidizing species. The amount of dissolved oxygen is much greater than the amount contained in the pressurized water of the PWR reactor. BWR fuel assemblies are very sensitive to this type of corrosion, and solutions that have been developed in the past to reduce or eliminate such localized corrosion include, for example, coating one of the components contained in the coating to electrochemically with other components. Compatible (see document US-A-2006/0045232).

It is an object of the present invention to provide a Zr alloy assembly for BWR fuel assembly that is affected by shadow corrosion as little as possible, while having the ability to withstand conventional types of corrosion in terms of mechanical properties in use. Satisfactory nature.

To this end, the present invention provides a shadow corrosion resistant zirconium alloy which can be used in a boiling water nuclear reactor fuel assembly assembly, which is characterized by: Its composition in weight percentage is as follows: Nb = 0.4% - 4.5%. Sn=0.20%-1.7%. Fe=0.05%-0.45%. Fe+Cr+Ni+V=0.05%-0.45%, and Nb≦9×[0.5-(Fe+Cr+V+Ni)]. S = trace -400ppm. C = trace -200ppm. Si = trace -120ppm. O=6o0ppm-1800ppm. The rest are Zr and impurities from the process; During construction, after the alloy is finally thermally deformed, the alloy is subjected to one or more heat treatments between 450 ° C and 610 ° C for a total of at least 4 hours, plus at least one at least 25 % cold rolling operation of the rolling ratio, and the heat treatment after the thermal deformation does not exceed 610 ° C; The final heat treatment operation is carried out for a period of from 1 minute to 20 hours between 450 ° C and 610 ° C.

. Preferably, the composition of the alloy in weight percent is as follows: Nb=0.8%-3.6%. Sn = 0.25% - 1.7%. Fe=0.05%-0.32%. Fe+Cr+Ni+V=0.05%-0.32%, and Nb≦9×[0.5-(Fe+Cr+V+Ni)]. S = 10ppm - 35ppm. C = trace -100ppm. Si = trace -30ppm. O=600ppm-1800ppm. The rest are Zr and impurities from the process.

The alloy is subjected to one or more cold rolling operations during construction, the cold rolling operation, before or between or between the heat treatment operations, and the heat treatment is between 450 A total of at least 4 hours is carried out between temperatures of °C to 610 °C.

The alloy may be partially or completely recrystallized.

The alloy can be in a state of stress relief.

The present invention also provides an assembly for a boiling water nuclear reactor fuel assembly characterized in that it is constructed of an alloy of the type described above.

The present invention also provides a boiling water nuclear reactor fuel assembly characterized in that it comprises an assembly of the above type, and at least some of which are galvanically connected to other components constructed from Ni or stainless steel based alloys. In the situation.

The invention also provides the use of a fuel of the above type in a boiling water nuclear reactor wherein the primary fluid contains up to 400 parts per billion (ppb) of dissolved oxygen.

The primary fluid also contains up to 50 milliliters per gram (mL/kg) of dissolved hydrogen.

The primary fluid also contains up to 50 ppb of zinc.

The primary fluid also contains chemicals that are added to reduce the corrosion potential of the material in contact with the primary fluid.

From the above, the present invention relates to a Zr alloy for a BWR fuel assembly assembly which contains significant amounts of Nb and Sn and a small amount of Fe. It may also contain limited amounts of Cr, Ni, V, S and O.

An essential requirement for such alloys to undergo one or more heat treatments between 450 ° C and 610 ° C for a total of at least 4 hours to ensure decomposition of the β Zr phase from the early heat treatment into β Nb phase. Any heat treatment after hot deformation must be carried out at not more than 610 °C. To perform a higher temperature treatment, the β Zr phase is recreated, which may reduce the corrosion behavior of the alloy.

One or more cold rolling operations may be performed before, and/or between, and/or after the heat treatment. In particular, heat treatment between 450 ° C and 610 ° C can be carried out between the cold rolling processes as an intermediate annealing. At least one of these cold rolling passes must be implemented with a reduction of at least 25%.

The operation of such heat treatment and cold rolling operations should be followed by a final heat treatment at a temperature not lower than 450 ° C and not higher than 610 ° C for a duration of from 1 minute to 20 hours. Experience has shown that the implementation of the aforementioned long heat treatment does not result in a compositional equilibrium between the β Nb and the Zr(Nb,Fe) ₂ precipitation phase even over a total duration of from 10 hours to 100 hours. At least one sufficient (≧25% reduction ratio) at least one cold rolling operation is performed between or after this/this isometric treatment, plus a final heat treatment (such as, for example, non-limiting example, stress relief or recrystallization) Annealing) can contribute to this balance or sufficiently close to this balance while maintaining reasonable processing time.

In such cases, the general defects of using Nb-containing alloys in BWRs can be overcome, and there is an environment in which components made of such alloys are placed in close proximity to each other in the composition of Ni-based alloy or stainless steel. Under the current connection condition, the shadow corrosion provided by the alloy is thereby.

It can be considered that when the Zr alloy component is in another medium constructed of a Ni-based alloy or stainless steel in a medium having a non-zero conductivity, in the case where exchange of electrons occurs, and when they are surrounded by The medium (primary fluid) contains up to 400 parts per billion dissolved oxygen, ie up to 50 ml / gram of hydrogen dissolved and / or up to 50 parts / billion parts of zinc, possibly containing added metals, methanol, When any other chemical used to reduce the corrosion potential of the material in contact with the primary fluid of the reactor, there is a current connection in the reactor. This is usually achieved when the components are separated by a distance of less than 20 mm.

Naturally, these alloys can also be used to make components for BRW fuel assembly, and their properties make them well suited for such applications without current connection.

The shading corrosion factor observed on the BWR fuel assembly assembly, as mentioned above, is caused by the illumination-assisted current connection phenomenon that occurs in the oxygen-containing medium. The special effects of irradiation are difficult to replicate in the laboratory, but it is known that irradiation accelerates the observed phenomenon. The role of oxygen and current linkages is easier to assess in laboratory tests using the following experimental procedure.

A sample of the inventive alloy and the reference alloy is placed in an autoclave under oxidizing conditions. Two samples were tested for each alloy, one linked to Inconel (Ni-based alloy) sheets, while the other is not joined. A dissolved oxygen content of 100 ppm was maintained in the medium, and a boron content of 0.12% in the form of boric acid, and a lithium content of 2 ppm in the form of lithium oxide. The goal is to obtain a high oxygen potential that affects the sample in the current-linked condition comparable to the time elapsed in the BWR.

The sensitivity of the alloy to shadow corrosion is indicated by the ratio between the thickness of the oxide formed on the bonded sample and the unbonded sample. The greater the ratio, the more sensitive the alloy is to the bond and therefore the more sensitive it is to shadow corrosion. It is believed that a ratio greater than 2.5 represents an alloy that is highly sensitive to shadow corrosion, making it unsuitable for use in a current-coupled state in a reactor.

A variety of tests have been performed and the results are summarized in the tables and figures below.

The effect of tin on the oxidation of tubes made of an alloy having 1% Nb and 0.1% Fe in lithium-containing water having 70 ppm Li at 360 ° C was evaluated on samples having the compositions listed in Table 1.

The following processing sequences were applied to all of these samples: Melting ingots; Forging into strips in the beta zone; Forging the strip in the alpha zone to have a diameter of 200 mm ( The form of the billet; Quenched from 1050 ° C in cold water; Drilling the billets; Squeeze after preheating to 600 ° C; and. Four cold-rolling passes in the pilger mill, each rolling ratio is in the range of 55% to 83%, and the passes are separated by annealing at 575 ° C for 2 hours. The final heat treatment was then carried out for 2 hours in the range of 560 ° C to 590 ° C to obtain a final tube having a diameter of 9.75 mm and a wall thickness of 0.57 mm.

Figure 1 shows the weight gain of tubes A through F after no current connection in the target medium for 112, 168, and 196 days. It can be seen that reference samples A and B each containing 0.039% and 0.19% Sn exhibit corrosion resistance in lithium-containing water, which begins to degrade between 112 and 168 days, and ranges from 168 days to 196 days. The inside is clearly flattened. Samples C to F of the present invention remained stable in corrosion during the same period. Therefore, the sample of the present invention needs to contain a Sn content of not less than 0.20%, preferably at least 0.25%, so that a region not subjected to shadow corrosion exhibits good corrosion behavior.

The sensitivity to shadow corrosion was tested on samples having the compositions and preparations listed in Table 2. These samples were subjected to recrystallization annealing at the end of the treatment.

Surprisingly, these tests show that in Zr-Nb-Sn-Fe alloys with Nb≧0.4% and Sn≧0.2%, reducing the Fe content to as low as 0.1% or even 0.06% does not cause shadow corrosion. Particularly sensitive alloy. The sensitivity to shadow corrosion becomes greater only below Fe = 0.05% (samples G and H) (i.e., using the criteria defined above, greater than 2.5).

It is believed that such advantageous utility can be attributed to the formation of intermediate Zr(Nb,Fe) ₂ precipitates or other intermetallic precipitates containing Nb and elements selected from Fe, Cr, Ni, and/or V, Different from the β Nb precipitate without these elements. The thermomechanical treatment according to the practice of the invention facilitates obtaining a balanced precipitate containing Nb in a sufficient number and reliably.

At the same time, the presence of beta Nb precipitates rather than beta Zr contributes to the preservation of resistance to uniform corrosion.

Figure 2 is a photomicrograph of a sample of the invention, i.e., sample L of Table 2, at a high magnification using a transmission electron microscope. It can be seen that the β Nb precipitate 1 and the representative intermetallic compound Zr(Nb,Fe) ₂ 2,3 of the present invention are present.

The presence of Fe also favors an alloy having about 3% Nb and 1% Sn, which is easier to recrystallize and thus gives the alloy a better conversion ability.

It has also been verified that oxygen at a concentration of 600 ppm to 1800 ppm and sulfur at a concentration of 10 ppm to 400 ppm have no effect on corrosion resistance and sensitivity to current bonding. Oxygen and sulfur can be added in a conventional manner, as set forth in documents FR-A-2 219 978 for oxygen and EP-A-0 802 264 for sulfur, to adjust the mechanical properties of the alloy, such as creep resistance.

Obviously, Sn does not have a significant influence on the sensitivity to shadow corrosion. Therefore, its content should be selected to achieve resistance to uniform corrosion and resistance to nodular corrosion (this tends to decrease, but not resistance to corrosion in lithium-containing media, and tends to improve mechanical properties) ) The purpose of coordination between. This coordination will vary with the application. Generally, the Sn content should be in the range of 0.2% to 1.7%, preferably in the range of 0.25% to 1.7%.

In addition, the difficulty of transition that may occur in certain alloys must also be considered.

As such, Fe should exceed 0.45%, otherwise precipitates of too large a size will appear.

Further, if the Nb content is too large (greater than 4.5%), the alloy hardens and the recrystallization is slowed down, especially when the Fe content is high, and thus, the precipitate containing both Fe and Nb becomes more Many and tend to anchor dislocations and grain boundaries.

Cr, V, and Ni, which cause precipitates similar to those formed by Fe, can be substituted for Fe and, therefore, from this point of view, must also be considered.

It has been found that the alloy of the present invention does not exhibit any particular transformation difficulties at a Nb content of less than 9 x [0.5-(Fe + Cr + V + Ni)], and preferably less than a Nb content of 9 x [0.4 - (Fe + Cr + V + Ni)]. Sex, including recrystallization.

However, if the Nb content is less than 0.4%, the resistance to nodular corrosion at 500 ° C may become insufficient.

Therefore, it is appropriate to select the Nb content in the range of 0.4% to 4.5%, and at the same time ensure that it satisfies the relationship Nb≦9×[0.5-(Fe+Cr+V+Ni)], preferably Nb≦9×[0.4 -(Fe+Cr+V+Ni)].

In addition, the alloy tube in the stress relief state is also tested for sensitivity to shadow corrosion. Their composition, the treatment applied to them, and the results of sensitivity to shadow corrosion appear in Table 3.

Annealing at 575 ° C for 2 hours after the last cold rolling operation constitutes the final annealing within the meaning of the present invention.

Sample W is not according to the inventors as it does not contain any Sn. However, it shows that Sn does not have any significant effect on shadow corrosion, at least when combined with other elements in the recommended amount.

It can be seen that for compositions that are compatible with the recrystallized sample, the stress relief sample is even less sensitive to shadow corrosion. The invention is thus compatible with both states and, therefore, is compatible with the intermediate state of partial recrystallization.

The superior performance of the BWR fuel assembly assemblies of the present invention allows them to be used in situations where shadow corrosion can be particularly high, such as when precious metals and/or iron and/or hydrogen are substantially dissolved in the water of the reactor.

1. . . N Nb precipitate

2,3. . . Intermetallic compound Zr(Nb,Fe) ₂

The invention may be better understood by reference to the following description of the accompanying drawings in which: FIG. 1 shows that Sn is contained in a zinc-containing water at 360 ° C using a Zr alloy containing 1% Nb and 0.1 Fe. The effect of oxidation in ).

Figure 2 is a photomicrograph of an alloy of the invention containing a representative precipitate of the alloy of the present invention.

Claims (11)

A shadow-resistant zirconium alloy for a boiling water nuclear reactor fuel assembly assembly characterized in that: ‧ its composition by weight percentage is as follows: ‧ Nb = 0.4% - 4.5% ‧ Sn = 0.20% - 1.7% ‧Fe=0.05%-0.45%‧Fe+Cr+Ni+V=0.05%-0.45%, and Nb≦9×[0.5-(Fe+Cr+V+Ni)]‧S=micro-400ppm‧C= Trace -200ppm‧Si=minor-120ppm‧O=600ppm-1800ppm‧The rest is Zr and impurities from the process; ‧ During the construction, after the final thermal deformation of the alloy, the alloy is applied at 450 °C One or more heat treatments to a temperature of 610 ° C for a total of at least 4 hours, plus at least one cold rolling operation of at least 25% rolling ratio, and the heat treatment after the thermal deformation does not exceed 610 ° C; The final heat treatment operation is carried out for a period of from 1 minute to 20 hours between 450 ° C and 610 ° C. According to the alloy of claim 1, wherein the composition of the alloy in weight percentage is as follows: ‧Nb=0.8%-3.6%‧Sn=0.25%-1.7%‧Fe=0.05%-0.32%‧Fe+Cr+ Ni+V=0.05%-0.32%, and Nb≦9×[0.5-(Fe+Cr+V+Ni)]‧S=10ppm-35ppm‧C=micro-100ppm‧Si=micro--30ppm‧O=600ppm -1800ppm‧ The rest are Zr and impurities from the process. An alloy according to claim 1 or 2, wherein the alloy is subjected to one or more cold rolling operations during construction, the cold rolling operation being before or between or between the (and the) heat treatment operations And the (equal) heat treatment is carried out for a total period of at least 4 hours between 450 ° C and 610 ° C. According to the alloy of claim 1 or 2, it is partially or completely recrystallized. According to the alloy of claim 1 or 2, it is in a stress removal state. An assembly for a fuel assembly of a boiling water nuclear reactor, characterized in that the assembly is constructed of an alloy according to any one of claims 1 to 5. A boiling water nuclear reactor fuel assembly characterized in that the assembly comprises an assembly as in claim 6 and at least some of the components are placed in a current with other components constructed from Ni or stainless steel based alloys. Under the link status. A use of a fuel as set forth in claim 7 in a boiling water nuclear reactor, wherein the primary fluid comprises up to 400 parts per billion of dissolved oxygen. The use according to item 8 of the patent application, wherein the primary fluid contains up to 50 ml/g of hydrogen. The use according to claim 8 or 9, wherein the primary fluid also contains up to 50 parts per billion of zinc. The use according to claim 8 or 9, wherein the primary fluid also contains a chemical added to reduce the corrosion potential of the material in contact therewith.